State Raoult's law.

Raoult's law states that "At a given temperature the relative lowering of vapour pressure of dilute solution containing non-volatile solute is equal to the mole fraction of solute in the solution".
$\frac{{\mathrm{P}}^0-{\mathrm{P}}_{\mathrm{s}}}{{\mathrm{P}}^0}={\mathrm{X}}_2$; where P⁰ = vapour pressure of pure solvent
P_s = vapour pressure of solution of non volatile solute
X₂ = mole fraction of solute